Carbon nanotubes (CNTs) are nano-sized carbon-based sorbents, which have high surface area, large aspect ratio, can be self-assembled and are known to be stable at high temperatures. It is therefore conceivable that separation techniques, such as, gas chromatography (GC) can benefit from their unique properties and nano-scale interactions. Self-assembly, in-contrast to packing these materials in a tube, prevents them from agglomeration and thus facilitates in retaining their nano-characteristics. In this research, novel substrates, such as, steel tubings, on a scaled-up level have been explored for the self-assembly process of CNTs, for applications such as gas chromatography, where the CNTs served as stationary phases.
In the first part of this research, the self-assembly of multi-walled carbon nanotubes (MWCNTs) on the inside wall of long stainless steel tubings was studied. The CNTs were deposited by the chemical vapor deposition (CVD) using ethylene as the carbon source and the iron nanostructures in the stainless steel as the catalyst. Variation in uniformity in terms of the thickness and morphology of the deposited film and surface coverage were studied along the length of a tube by scanning electron microscopy (SEM). The effects of process conditions, such as flow rate and deposition time on the coating thickness, were studied. The catalytic effect of the iron nanostructures depended on surface conditioning of the tubing. It was found that the pretreatment temperature influenced the quality of the nanotube coating. The morphology of the CNT deposit supported the base-growth scheme and VLS (vapor—liquid—solid) growth mechanisms of CNTs. This study served as the basis for the development of CNTs in the larger scale application. Scaled up self-assembly of single-walled carbon nanotubes (SWCNTs) was studied in long tubes and finally they were used as GC columns.
The strategy for selective SWCNT growth required the prevention of iron in the bulk steel from participating in the catalytic CVD process, as the presence of iron always led to MWCNT formation. Consequently, silica lined stainless steel tubings, such as, SilcosteelTM and SulfinertTM were selected. A SWCNT film with an average thickness of 300 nm was self-assembled by a unique single-step, catalytic CVD process consisting of dissolved cobalt and molybdenum salts in ethanol, where ethanol served as the precursor and cobalt and molybdenum as catalysts. Such large-scale assembly required process and catalyst optimization. A variety of organic compounds with varying polarity were separated at high resolution and the column efficiency demonstrated around 1000 theoretical plates/in, comparable to commercial GC columns. Evaluation of Van’t Hoff and Van deemter plots suggested that the CNTs followed classical chromatography behavior. Comparison of capacity factors (k’) and isosteric heats of adsorption (ΔHS) with a packed column containing a commercial sorbent (Carbopack CTM) showed comparable results. This demonstrated high capacity and strong sorbate-sorbent interactions on the SWCNT phase. Evaluation of the McReynolds constants suggested that the SWCNT was a non-polar phase. The high surface area of the SWCNT media allowed separations of gases, and at the same time, its high thermal stability (>425°C) permitted separations of higher molecular weights at higher temperatures, thus extending the range of conventional chromatography on the same column.
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